WO1996028268A1

WO1996028268A1 - Refractory systems

Info

Publication number: WO1996028268A1
Application number: PCT/GB1996/000624
Authority: WO
Inventors: Stephen John Lee
Original assignee: Didier-Werke Ag
Priority date: 1995-03-15
Filing date: 1996-03-15
Publication date: 1996-09-19
Also published as: GB9505206D0; AU5113396A

Abstract

A continuous casting machine includes two casting rollers (2) which define a gap communicating with a reservoir of molten metal. Urged into sliding contact with a proportion of the area of the end surfaces of the casting rollers are respective side dams (3). Particulate solid refractory lubricant material which is in flowable form but has no added liquid is applied to the end surfaces of the casting rollers at positions shortly before they come into sliding contact with the side dams. The lubricant material occupies the interface between the casting rollers and side dams and seals it from the ingress of molten metal and protects the sliding surfaces from abrasion.

Description

REFRACTORY SYSTEMS

The present invention relates to refractory systems and is concerned with that type of system which includes a refractory plate or other member in sliding contact with a movable member. The invention is particularly though not exclusively concerned with such systems in which only a proportion of the surface of the movable member is in contact with the refractory member at any one time, the contacting surface portion constantly altering, that is to say moving repeatedly into and out of contact with the refractory member. The invention also relates to a method of sealing an interface between a refractory member and a movable member from molten metal in such a refractory system.

A typical example of such a system is a continuous strip casting machine which includes two cooled rollers which cooperate to define a gap and which are rotated in opposite senses with the upstream end of the gap communicating with a reservoir of molten metal . Metal solidifies on the surface of the rollers and the two solidified areas are pressed together in the nip of the rollers to form a unitary strip which is discharged continuously from the downstream end of the gap. A proportion of the area of each adjacent pair of ends of the rollers is in sliding engagement with a refractory- plate or side dam. The side dams are typically spring biassed into contact with the ends of the rollers in an attempt to seal the gap. Alternatively the end surfaces of respective end plates engage the curved surface of each roller along its entire length. In either case there is a number of conflicting requirements in that the refractory material must be compatible with the molten metal, should preferably have a low tendency to wetting by the molten metal, should not interfere with the thermal balance of heat extraction and should neither abrade, nor be abraded by, the movable member.

Much research and development has been conducted to optimise the material composition and design configuration and to develop automatic pressure sensing systems but the sliding interface remains one of the areas of weakness in many continuous casting machines. If molten metal penetrates into the interface it tends to solidify and cause abrasion and/or mechanical failure and can ultimately result in a catastrophic breakout of molten metal.

German Patent No. 4331055 discloses a continuous casting machine including side dams spring-loaded into contact with the end surfaces of the casting rollers. The inner surface of each side dam is provided with an arcuate area or a plurality of discrete areas of a solid permanent lubricant such as boron nitride or PTFE whose purpose is to seal the sliding interface. However, the solid lubricant is progressively abraded away by contact with the moving end surfaces of the rollers and the abraded lubricant inevitably ends up in the molten metal and contaminates it to a greater or lesser extent . Once the area of solid lubricant has been worn down to the level of the surface of the side dam, operation must be interrupted and the molten metal drained away to permit the side dams to be removed and replaced by new ones or at least the solid lubricant to be replaced.

German Patent No.4002731 discloses a continuous casting of generally similar type in which lubricant material is injected into the interfaces between the ends of the casting rollers and the side dams in an attempt to overcome the problems referred to above. The lubricant material comprises boron nitride or the like mixed with rapeseed oil to form a flowable composition. However, it is found in practice that the rapeseed oil is subject to thermal decomposition or cracking by the heat of the molten metal and that the decomposition products result in clogging of the surfaces defining the interface whereby the desired lubricant effect is not in fact achieved.

It is the object of the invention to provide a refractory or metallurgical system of the type referred to above in which a reliable seal is produced between the refractory member and the movable member thereby preventing ingress of molten metal between their cooperating surfaces and thus eliminating or reducing wear of the cooperating surfaces whilst eliminating the need for periodic shutdown of the system to permit the refractory member to be removed and/or the lubricant supply to be replaced. It is a further object of the invention to provide a method of sealing an interface between a refractory member and a movable member in such a system from the ingress of molten metal.

According to the present invention there is provided a refractory system, e.g. metallurgical apparatus and particularly a continuous casting machine, including a refractory plate or other member in sliding contact with a proportion of the area of a movable member and means for applying refractory lubricant material in flowable form, e.g. in the form of a fluid or paste to the surface of the movable member, e.g. at a position before that at which it comes into contact with the refractory plate, the lubricant material comprising solid refractory lubricant with no added liquid.

According to a further aspect of the present invention a method of sealing an interface between a refractory member and a movable member in a refractory or metallurgical system, particularly a continuous casting machine, from molten metal comprises applying lubricant material comprising only solid refractory lubricant with no added liquid to the surface of the movable member.

Accordingly the method and apparatus of the present invention makes use of solid refractory lubricant material with no added rapeseed oil or other inorganic liquid which is therefore not subject to thermal degradation. In this connection it has been found that fine particulate boron nitride, graphite or the like acts as a fluid or paste, that is to say may be caused to flow through a narrow pipe or the like, by the application of pressure to it due to its tribological characteristics.

The lubricant material preferably comprises boron nitride and/or graphite though other refractory lubricant materials may also be used provided that they are not wetted by molten metal, e.g. molten steel and are stable under oxidising and non-oxidising conditions at the temperature of the molten metal.

The lubricant material may be applied to the surface of the movable member in a strip extending transverse to its direction of movement in which case it will occupy a substantial proportion or all of the interface between the refractory member and the movable member or it may be applied at one or more discreet areas in which case the lubricant will be present in one or more strips in the interface, which strips will necessarily be of arcuate shape if the movable member moves in rotation rather than linearly. If the strip or one of the strips of lubricant material is arranged to be at or close to the communication of the interface with the molten metal it will prevent the ingress of metal into the interface. Due to the fact that the lubricant is in pasty or flowable form, rather than solid form, it will reliably fill the space between the moving surfaces and thus form a reliable seal.

Thus in the apparatus in accordance with the invention the lubricant material is applied to the surface of the moving member whilst it is in contact with the refractory member or at a position shortly before it comes into contact with the refractory member. After the relevant portion of the movable member moves out of contact with the refractory member the lubricant material may run or fall off the movable member, at least partially, and be collected. However, some or all of it may remain adhered to the movable member and this may be left in situ to come into contact again with the refractory member on the next occasion that portion of the surface of the movable member comes into contact with the refractory member, optionally supplemented by additional lubricant material applied by the applying means. However, the remaining lubricant material is likely to have been contaminated and/or have had its characteristics altered by contact with the molten metal and it is therefore preferred that the remaining lubricant material is positively removed from the surface of the movable member, e.g. by a scraper contacting the surface or by an air or water blast nozzle. Fresh lubricant material is then applied to the surface when or before it is again in contact with the refractory member. The characteristics of the lubricant material may be modified to facilitate its easy removal, e.g. by the addition of a material, such as pyrophyllite, which expands under the action of heat, and thus causes the lubricant material to lift away from the surface of the movable member after it moves out of the contact zone in which it is in contact with the refractory member.

Further features and details of the invention will be apparent from the following description of one specific embodiment which is given by way of example only with reference to the accompanying highly schematic drawing of a continuous casting machine.

The continuous casting machine comprises a metal reservoir which accommodates a pool of molten metal 1, in this case molten steel, and which is defined by two water-cooled copper casting rollers 2 and by two refractory side dam plates 3, each of which is spring- loaded into engagement with approximately one quadrant of the surfaces of two adjacent ends of the rollers. The rollers 2 define a small gap between them and are contra- rotated in the directions indicated by the arrows. The cooled rollers cause the metal to solidify on the surfaces of the rollers in the areas indicated by the arcs 4 and as the rollers rotate the solidified portions are forced together at the nip or pinch point 5 of the rollers and are discharged downwards in the form of a unitary solidified strip 6. The problems described above of compatibility between the steel, the edges of the rollers and the side dams occur within the arcs of contact 4. Once the solidified strip 6 has broken free of the rollers 2 and those portions of the rollers which were in contact with the molten steel are no longer in contact with it, the associated portions of the contact rims 7 of the rollers are cooled until they again come into contact with the melt.

In accordance with the invention, each roller 2 is associated with a solid lubricant dispenser, constituted in this case by a respective nozzle 8 positioned opposed to the rim 7 at each end of the roller at a position immediately upstream of that at which the rim comes into contact with the associated side dam. The nozzles 8 are connected by means of pipes to a pump or the like (both not shown) which dispenses solid lubricant into contact with the end surfaces of the rollers. The lubricant preferably comprises boron nitride or graphite in finely ground form which is flowable without any additives and has a pasty composition and optionally one or more other substances to modify its characteristics or consistency or its ability to be subsequently removed from the rollers. The lubricant adheres, at least for the most part, to the surfaces of the rollers and is carried into the interface between the rollers and the side dams in the form of an arcuate strip where it forms a seal with the opposed surfaces defining the interface and prevents the ingress of molten steel. After the strip of lubricant moves out from the interface some of it may fall or run down under the action of gravity but that proportion of it which remains adhered to the rollers is positively removed at a position upstream of the dispensing nozzles by lubricant removal means, in this case respective scrapers 9. The lubricant removed falls down, e.g. into waste containers and is then discarded. The lubricant material thus not only provides the correct tribological conditions at the interface but also seals the interface and prevents abrasion of the opposed surfaces by metal which has penetrated into the interface. The side dams will need to be removed less frequently and the output efficiency of the casting apparatus is therefore increased and the maintenance cost decreased.

In the described embodiment the lubricant is supplied through nozzles situated upstream of the side dams. However, it could of course also be dispensed through passages provided in the side dams themselves which terminate in dispensing orifices, which may be circular or in the form of radial or peripheral slots and which are preferably situated at positions upstream of those at which the rollers come into contact with the melt.

In both cases the pump or the like which supplies the lubricant material may be linked to the roller drive mechanism or its control system so that the rate of lubricant supply is related to the speed of rotation of the rollers so that the rate of lubricant supply may always be at the optimum value.

In the embodiment described above the cast strip is discharged downwardly and the pool of molten metal is received in the space defined by the rollers and side dams. However, the invention is equally applicable to other types of continuous casting machine, e.g. of the type in which the space for the pool of metal is partially defined by side walls whose lower edges engage the peripheral surfaces of the rollers and in this case the invention may be applied to the roller/side dam interfaces and/or the roller/side wall interfaces. The invention is also applicable to those casting machines in which the rollers are arranged one above the other and the cast strip is discharged generally horizontally.

In a further alternative system the metal is cast through a slot onto a cooled moving belt where it solidifies between stationary retaining walls whose underside is in sliding contact with the belt. The interface between the belt and the walls are subjected to the same heat and abrasion conditions as described above in relation to the side dams of a roll type caster and the application of a flowable refractory lubricant material to the belt before it comes into contact with the walls will improve the seal and result in more efficient operation. The lubricant material may again optionally be removed from the belt after it has moved out of contact with the walls.

Claims

1. A refractory system including a refractory member (3) in sliding contact with a movable member (2) to define therebetween an interface which is exposed, in use, to molten metal and means (8) for applying lubricant material in flowable form to the surface of the movable member (2) characterised in that the lubricant material comprises only solid refractory lubricant material with no added liquid.

2. A system as claimed in claim 1 in which the refractory member (3) is in sliding contact with only a proportion of the area of the movable member (2) .

3. A system as claimed in claim 2 in which the means

(8) for applying lubricant material is situated to apply the lubricant material to the movable member (2) at a position shortly before it moves into sliding contact with the refractory member (2) .

4. A system as claimed in claim 2 or claim 3 including means (9) cooperating with the movable member (2) at a position after it moves out of contact with the refractory member (3) and arranged to remove solid lubricant material from the surface of the movable member (2) .

5. A system as claimed in any one of the preceding claims in which the solid lubricant material is boron nitride or graphite.

6. A system as claimed in any one of the preceding claims which is constituted by a continuous casting machine, the movable member (2) constituting a casting roller and the refractory member (3) constituting a side dam in sliding contact with an end surface of the casting roller.

7. A method of sealing an interface between a refractory member (3) and a movable member (2) from molten metal comprising applying lubricant material in flowable form to the surface of the movable member (3) characterised in that the lubricant material comprises only solid refractory lubricant material with no added liquid.

8. A method as claimed in claim 1 in which the refractory member (3) is in sliding contact with only a proportion of the area of the movable member (2) .

9. A method as claimed in claim 8 in which the lubricant material is applied to the movable member (2) at a position shortly before it moves into sliding contact with the refractory member (2) .

10. A method as claimed in claim 8 or 9 which includes removing the lubricant material from the movable member (2) at a position after it moves out of contact with the refractory member.

11. A method as claimed in any one of claims 7 to 10 in which the solid lubricant material is boron nitride or graphite.

12. A method as claimed in any one of claims 7 to 11 in which the movable member is a casting roller (2) of a continuous casting machine and the refractory member is a side dam in sliding contact with an end surface of the casting roller.